As is well known to those skilled in the paper-making art, a web is dewatered by a pressing pressure applied in a press section of a paper-making machine. For example, it is known in the prior art to provide a web pressure-dewatering means and web transfer means, in the pressure section of a paper-making machine, in the form of a combination of a pair of press rolls with a pair of needled felts therebetween. More recently, a variety of means for imparting pressure onto a web have been provided with a combination of a single press roll and a shoe in order to improve the dewatering efficiency through an increase in the nip width, under application of a pressure, on the web. One example of the latter combination is known as a shoe-press assembly, whose simplified construction is illustrated in FIG. 7(a)
Referring to the prior art arrangement shown in FIG. 7(a), a shoe press assembly 20 includes a press roll 21 and a shoe 22, wherein the shoe 22 is shaped so as to conform to the press roll 21 along its circumference. In this way a nip width N under an applied nip pressure is large when compared to an arrangement employing a pair of press rolls. A shoe-press belt 23, that is constructed of a high-molecular elastic member, e.g., such as urethane, and a base cloth layer, runs between press roll 21 and shoe 22. A web P runs between shoe-press belt 23 and press roll 21, so as to be sandwiched between upper and lower needled felts 24 and 25. As a consequence, web P is press-dewatered, under the nip-pressure produced between press roll 21 and shoe 22, with the squeezed-out water migrating into upper and lower needled felts 24 and 25.
In this situation, however, as the water migrates into the upper and lower needled felts 24 and 25, they expand in sections where the felts are relieved of the nip pressure. As a result, the water accumulated in felts 24 and 25 actually migrates back into web P, by capillary action, thus rewetting the paper and causing a reduction in the efficiency of the dewatering process.
In order to solve the rewetting problem, a shoe press assembly 20', as illustrated in the prior art arrangement shown in FIG. 7(b), has been proposed, and which is disclosed in U.S. Pat. No. 4,483,745. In the shoe press assembly 20', a web P is sandwiched between a single needled felt 24 and a shoe press belt 23 and dewatered under application of a nip pressure produced by a press roll 21 and a shoe 22, such that the squeezed-out water migrates into needled felt 24. This arrangement still rewets the paper, since even though needled felt 24 is only a single piece, a portion of the water absorbed in sections of the felt having lower applied pressure still migrates back to the web.
Also in the prior art arrangement shown in FIG. 7(b), the web P of shoe-press assembly 20' and received on shoe press belt 23 has suffered from difficulties associated with paper transfer to and reception by a next step in the process. In particular, the surface of shoe press belt 23 is often highly polished so as to be very smooth. As a result, a water film of a uniform thickness is produced in a clearance between shoe press belt 23 and web P, whereby web P strongly adheres to shoe press belt 23 in the presence of the water, resulting in very poor paper release.
As a technique to improve the paper release, two concepts have recently been proposed. A first one, as illustrated in the prior art arrangement shown in FIG. 8(a), is a shoe press belt 23 having a web-receiving face 26b, and on the surface of which are formed many of protrusions 27. A second one, as illustrated in the prior art arrangement shown in FIG. 8(b), is a shoe press belt 23 also having a web-receiving face, and on the surface of which are formed many of recesses 28'. The first one, as illustrated in the prior art arrangement shown in FIG. 8(a), is a technique disclosed in Japanese patent document JP-A-94-57678. In this prior art technique particulate filler 27, such as kaolin clay, inorganic material, polymer material and metal, which have a higher hardness than a high-molecular weight elastic member 26, is mixed into the high-molecular weight elastic member 26, reinforced by a base member 26a, and a web-receiving face 26b is polished to make particles of the particulate filler 27 protrude from the surface of the web-receiving face 26b, thus producing many of protrusions 27' on web-receiving face 26b. The protrusions 27' function to break-up the water film that is easily formed between web P and web-receiving face 26b, thus decreasing the adhesive force between web P and web-receiving face 26b of shoe press belt 23. This result in the web being transferred to or received by a next step in the process with ease.
The second one, as illustrated in the prior art arrangement shown in FIG. 8(b), is a technique disclosed in U.S. Pat. No. 4,552,620. This technique is such that when a high-molecular weight elastic member 26, that is reinforced by a base member 26a, is formed, a portion of the upper surface of the base member 26a is spray coated so that many of closed isolated bubbles 28 are formed. High-molecular weight elastic member 26 and a web-receiving face 26b are then polished so as to form a very large number of recesses 28' by the opening of the originally closed bubbles 28, at the surface. The recesses 28' function to break the water film that is formed between web P and web-receiving face 26b, and also decrease the adhesive force between web P and web-receiving face of the belt 23, such that the web is transferred to and received by a next step in the process with ease.
However, in order to attain the above-described effect, it is required to mix particulate filler 27 into a high-molecular weight elastic member 26 at a content ranging from 30 to 40 percent weight (wt %). Particulate filler 27 is harder than the high-molecular weight elastic resin so that belt 23 itself comes to have a high hardness, which results in easy cracking, especially at edge portions of a shoe, thereby decreasing the durability of the belt. In addition, since particulate filler 27 is mixed in the range of 30 to 40 wt % in content, the weight of belt 23 increases, which is problematic to operations, such as, looping belt 23 over in a shoe press assembly.
In addition, many hours are required to manufacture a high-molecular weight elastic member into which closed bubbles are mixed by, e.g., spray jetted synthetic resin. Also, closed bubbles once formed in the synthetic resin mass disappear over time due to fluidity of the resin resulting from conventional spraying methods. Thus, while there is a need to raise the viscosity of the sprayed resin, as the viscosity of the spraying resin is higher, spraying of the resin in a stable manner is harder to achieve and patches of coating are produced more easily, with the result that many kinds of problems such as unevenness of bubble size occur.